The present invention relates to a frame pulse generation apparatus for a standard definition video cassette recorder (SD-VCR), and more particularly, to a frame pulse generation apparatus which generates for a correct frame pulse to form an accurate picture in a SD-VCR, even though an error occurs in a playback signal.
Development of a semiconductor technology has enabled digital signal processing to be applied to fields of wide-band video and audio recording and reproducing, among others. In addition, development of a recording and reproduction medium and recording and reproduction technology has enabled general home VCRs to utilize digital signal processing.
FIGS. 1A and 1B illustrate a relationship between a track and a frame in a SD-VCR system, one-frame data is recorded in ten tracks in a 525/60 system of NTSC and in twelve tracks in a 625/50 system of PAL. That is, assuming that a number is assigned to each track, one-frame data is recorded in the 0th to 9th tracks on a video tape in a NTSC system, and in the 0th to 11th tracks in a PAL system. Thus, when a signal is reproduced from a video tape, the 0th to 9th tracks in the NTSC system and the 0th to 11th tracks in the PAL system should be contained in a one-frame pulse. If the phase of the frame pulse with respect to the one-frame tracks deviates, a normal picture cannot be formed.
Referring to FIG. 2, four sectors such as an insert and track information (ITI) sector, an audio sector, a video sector and a sub-code sector are arranged in each track. Among them, the audio sector and video sector include a total of 169 sync blocks. Each sync block includes 90 bytes in which sync areas SYNC0 and SYNC1 occupy 2 bytes, identification (ID) codes ID0, ID1 and ID2 occupy 3 bytes, and data fields DATA0-DATA84 occupy 85 bytes. A sync pattern is inserted into the sync area. An ID includes a 2-byte ID data and a 1-byte ID parity. Information representing a current track and the track pair number, which represents a pair of two neighboring tracks as 4 bits, are recorded in an area indicated by ID0 and ID1. Also, all sync blocks in a track have the same track pair number, which varies for every other two tracks. Table 1 shows a relationship between a track pair number and a track.
TABLE 1 ______________________________________ Track Pair No. 525/60 System 625/50 System ______________________________________ 0 Track 0 & 1 Track 0 & 1 1 Track 2 & 3 Track 2 & 3 2 Track 4 & 5 Track 4 & 5 3 Track 6 & 7 Track 6 & 7 4 Track 8 & 9 Track 8 & 9 5 Reserved Track 10 & 11 ______________________________________
Meanwhile, when reproducing a playback signal, a head is not controlled with respect to which track should be read first among the various tracks. Accordingly, it is necessary to form one-frame of data within a one-frame pulse without being out of phase, using read-out data.
FIG. 3 shows a flowchart for explaining a frame pulse generation operation in a conventional SD-VCR. An ID code is detected among the reproduced data in step 1, and a track pair number is detected from the detected ID code in step 2. In step 3, the detected track pair number is compared with a reference number "N" and a determination is made as to whether the former is consistent with the latter. Here, the reference number "N" is 4 in case of the 525/60 NTSC system, and 5 in case of the 625/50 PAL system. In step 4, transition of a frame pulse level is performed whenever the detected track pair number is consistent with the reference number "N." Since the track pair number is detected in sequence of "0011223344" in NTSC systems, transition of a frame pulse level is performed at the time when a track pair number "4" ends or "0" starts. Since the track pair number is detected in sequence of "001122334455" in PAL systems, transition of a frame pulse level is performed at the time when a track pair number "5" ends or "0" starts. If the detected track pair number is inconsistent with the reference number "N," the program returns to step 1 to perform the steps 1-4 repetitively. That is, transition of the frame pulse level is performed everytime the track pair number becomes "N" as shown in FIG. 4A. As a result, the frame pulse is subject to level-transition for every 10 tracks in NTSC systems and for every 12 tracks in PAL systems.
However, when a detected track pair number differs from an original one due to a detected ID code which contains an error, a level transition does not occur at the time (N') where the level transition should occur, as shown in FIG. 4B. In this case, accurate signal processing to form a normal picture is not possible.